WASHINGTON (TNS) — For two years now, Sens. Rob Portman and Claire McCaskill have been waging a war against a website that one watchdog called "the Wal-Mart" of child sex trafficking.

On Tuesday, they — along with more than 20 other senators from both parties — began the process of adding a new tool to their arsenal.

The duo introduced a bill that they hope will undermine what has been a cornerstone of Backpage.com’s legal defense as it has faced repeated lawsuits from families of underage girls who were sold for sex on that website.

Backpage.com, a sprawling online marketplace, has become so well-known for its sex trafficking that the National Center for Missing and Exploited Children reported that it first goes to that site when a child is reported missing.

But Backpage has successfully defended itself in a spate of lawsuits from parents enraged that their children were trafficked on the site. Citing the 2006 Communications Decency Act, they have successfully argued that they are protected by a provision of the law that protects Internet publishers from content created by third parties.

Doing so might, for example, protect Facebook from someone who posted video of a crime, or from libel. But it has also given Backpage a defense as it continues to to post the ads.

Portman and the other senators argue that the law was never intended to protect websites that help enable sex trafficking. Their bill would clarify section 230 to ensure that websites that are essentially sex trafficking marketplaces can be held liable. It does so by allowing victims of sex trafficking to take websites that enable sex trafficking to court; by eliminating federal liability protections for websites that assist, support or facilitate a violation of federal sex trafficking laws and by allowing state police — not just the Department of Justice — to crack down on people or businesses that violate federal sex trafficking laws.

In a statement, the GOP senator called stopping trafficking “one of the great humanitarian and human rights causes of the 21st century.”

“For too long, courts around the country have ruled that Backpage can continue to facilitate illegal sex trafficking online with no repercussions,” he said. “The Communications Decency Act is a well-intentioned law, but it was never intended to help protect sex traffickers hwo prey on the most innocent and vulnerable among us.”

Portman, the chairman of the Senate Permanent Subcommittee on Investigations, and McCaskill, the ranking Democrat on that subcommittee, have held hearings and conducted an 18-month investigation that resulted in a report finding that Backpage.com knew it was enabling child sex trafficking and knowingly concealed evidence of criminality by systematically editing its “adult” ads to mask the fact that the site was selling children for sex.

Since 2007, the National Human Trafficking Hotline has received reports of 22,191 sex trafficking incidents -- 5,551 alone in 2016. And the National Center for Missing and Exploited Children reported an 846 percent increase in reports of suspected child sex trafficking from 2010 to 2015 — an increase that the organization linked to the increased use of the internet to sell children for sex. The site has estimated annual revenue in excess of $150 million.

Portman and McCaskill subpoenaed Backpage CEO Carl Ferrer in November 2015 to address the subcommittee; when he ignored that subpoena, the Senate passed a civil contempt resolution to authorize a vote against Backpage -- the first time such a legal action had been taken in 20 years.

In October, Ferrer was arrested in Houston on a California warrant, his company accused of knowingly taking money from underage prostitutes.

Sen. Sherrod Brown, D-Ohio, signed onto the bill Tuesday.

“We need to bring all traffickers to justice — no matter how they carry out this heinous crime,” he said. “With evolving technology, we must ensure the law keeps pace with this modern-day slavery.”